Flowforming is an advanced forming process for the manufacture of hollow components that allows for the production of dimensionally precise and rotationally symmetrical metallic components. In production, flowforming processes are conducted at temperatures below the recrystallization temperature of the metal being flowformed. In other words, flowforming is usually a cold-forming process.
Subjecting a portion of metal to a flowforming process typically requires that the metal first be formed into a hollow preform that will fit onto the flowforming mandrel. Once fitted on the mandrel, the preform is then subjected to the flowforming process and shaped by compression with one or more hydraulically driven rollers applied to the outside diameter of the preform. To date, over fifty different types of metal and metal alloys have been successfully flowformed. To date, however, flowforming certain types of titanium alloys has not been possible at temperatures below the re-crystallization temperature.
Titanium can exist in two crystallographic forms. At room temperature, titanium has a hexagonal close-packed crystal (hcp) crystal structure, known as “alpha” phase (α phase). At around 883° C. (˜1,621° F.), the α phase transforms to a body-centered cubic (bcc) crystal structure, called “beta” phase (β phase). In titanium alloys, both the α and β phases may coexist over a range of temperatures. The lowest temperature at which a titanium or titanium alloy is completely converted to the β phase is known as the beta transus (β-transus).
Titanium alloying elements can raise or lower the β-transus temperature, and the elements are often classified based upon how they affect the β-transus temperature. “Alpha stabilizers” (α stabilizers; e.g., aluminum, carbon, gallium, germanium, nitrogen, and oxygen) tend to increase the temperatures where the α phase is stable, while “beta stabilizers” (β stabilizers; e.g., nickel, molybdenum, and vanadium) tend to suppress the β-transus temperature thereby allowing the β phase to remain stable at lower temperatures.
In discussing the metallurgy of titanium, it is common to separate titanium alloys into five categories, referring to the common phases present: alpha alloys (α alloys), near-alpha alloys (near-α or superalpha alloys), alpha-beta alloys (α-β alloys), near-beta alloys (near-βalloys), and beta alloys (β alloys). These alloy categories describe the origin of the microstructure in terms of the basic crystal structure favored by an alloy composition.
At temperatures below the β-transus temperature, an a alloy has no β phase. A near-α alloy generally includes only limited β phase at temperatures below the β-transus, and so it may appear microstructurally similar to an α alloy at lower temperatures, while an α-β alloy will include both an alpha phase and a retained or transformed beta phase. Both near-β alloys or β alloys tend to retain the β phase on initial cooling to room temperature.
α-β alloys are heat treatable to varying extents and most are weldable with the risk of some loss of ductility in the weld area. These are generally medium to high strength materials with tensile strengths generally in the range of from about 120,000 psi (˜830 MPa) to about 181,000 psi (˜1250 MPa) and with useful creep resistance up to about 350 to 400° C. Hot forming qualities are generally good, but traditionally the α-β alloys could not be readily formed at room temperature. The α-β alloys have high yield point to tensile strength ratios, usually over 90%, resulting in a very high strength with limited ductility.
This low ductility or low elongation limits the α-β alloy's plastic formability to a very narrow range, rendering α-β alloys unsuitable for use in many traditional cold-forming processes (e.g., flowforming). For example, M. Koch, et al. were able to produced seamless Ti6Al-4V titanium tubes using a flow-forming process only by conducting the flow-forming process at temperatures above the recrystallization temperature of the titanium alloy. This procedure for flow-forming at temperatures above the recrystallization temperature is not economical or practical because the hot temperature damages equipment. Also, this high-temperature flow-forming process is not capable of producing dimensionally precise tubes. The higher high-temperature flow-forming process demands that the tube being flowformed have relatively thick walls. Also, the tubes undergo significant dimensional changes as they are cooled to room temperature. Additional processing (e.g., secondary machining) is needed to produce the desired shape and/or dimensions.
In the past, flowforming α-β titanium tubes has been problematic or impossible, with the α-β titanium preforms consistently cracking during the flowforming processes. Because of this, flowforming processes have not been an acceptable manufacturing method of producing α-β titanium alloys. A need exists in the art for new methods that allow flowforming of α-β titanium alloys.